1. The Field of the Invention
The present invention pertains to the measurement of average downhole steam quality and in particular to the measurement of the velocity of the two phases by sensing tracers, each having a single phase affinity, over a measured distance.
2. The Prior Art
Steam flooding has become an accepted practice for recovery of petroleum products from marginal fields or reservoirs that require a degree of stimulation to produce a satisfactory flow of crude petroleum. There is a need for a simple method and apparatus to determine the quality of steam at the wellhead of an injection well. Such a measurement, if simplified, would be particularly useful in determining the amount of heat which is applied to the underground reservoir by the injected steam.
The measurement or monitoring of steam quality is important since the steam's quality and hereby its reservoir or formation heatup effect directly affects the resulting production operations. Further, the quality of the steam which can be most economically injected into a particular substrate or reservoir is contingent on a number of circumstances. The latter include the age of the reservoir and the anticipated prospects for extracting commercially justified amounts of hydrocarbon products therefrom.
In brief, it is desirable that the quality of steam which is injected into each injection well be altered or adjusted to a level of quality that best conforms to the condition of the formation penetrated by that well. Clearly the quality of the steam must be known before any alteration or adjustment can be made.
It is known that in order to be particularly effective in this type of stimulation operation, the flow of injected steam must be monitored by use of metering means positioned in the steam-carrying line adjacent the wellhead. It can be appreciated that steam will normally leave the steam generator or source at a 15 known quality, pressure and mass flow rate. As the pressurized steam flow progresses towards an injection well, however, the quality will usually be substantially decreased. A decrease in the quality can be based on such factors as the distance between the well and the source and the effectiveness of pipe insulation. It will further depend on the pipe layout including number and orientation of fittings through which the steam has to travel prior to reaching the injection port or well because of phase separation that can occur in these fittings.
It is important, therefore, as a matter of economic practicality that a flow monitoring and controlling means be instituted into the steam-carrying conduit immediately upstream of each injection wellhead. A choke mechanism in the steam line will function to constrict the steam flow to thereby allow regulation of the steam mass flow rate which enters that particular well.
U.S. Pat. No. 4,836,032 discloses the use of an orifice plate in series with a critical flow choke to provide a method of measurement for both steam quality and mass flow rate. The present invention is distinguished from this earlier invention by the fact that only the steam quality is determined. The steam quality is determined by determining the slip ratio of the average vapor phase velocity to the average liquid phase velocity. The slip ratio is a function of the steam quality.
In 1962, Vance established that the slip ratio K of steam is related to the steam quality X.degree.. ##EQU1## and K=K (X.degree.), Vance's data can be approximately correlated as EQU K=(X.degree.).sup.-0.398,
It is believed that the steam pressure could play a role in the slip ratio i.e. the exponent on the "X.degree." should be a function of a steam pressure or dimensionless pressure EQU p/p.sub.c,
(p.sub.c -critical pressure of steam=3198.81 psia).
In a recent publication, Deichsel and Winter presented experimental data on slip ratio of critical flow of an air-water mixture and presented the slip ratio as function of quality (% of air in the mixture). Their results showed that for a quality between 10% and 100%, the slip ratio increases as the quality is reduced. They further showed that for the same quality, the slip ratio is reduced as the pressure increases. However, at a quality higher than 50%, the effect of pressure on slip ratio becomes less significant.
It should be pointed out that Deichsel and Winter's work used air and water for the two phases. There slip ratio is calculated according to an equation which showed the slip ratio as function of pipe area, density, fluid rate of the two phases and the Exit pressure and impulse force of the exit jet.